Simulation studies of current transport in metal–insulator–semiconductor Schottky barrier diodes

The current–voltage characteristics of Schottky diodes with an interfacial insulator layer are analysed by numerical simulation. The current–voltage data of the metal–insulator–semiconductor Schottky diode are simulated using thermionic emission diffusion (TED) equation taking into account an interfacial layer parameter. The calculated current–voltage data are fitted into ideal TED equation to see the apparent effect of interfacial layer parameters on current transport. Results obtained from the simulation studies shows that with mere presence of an interfacial layer at the metal–semiconductor interface the Schottky contact behave as an ideal diode of apparently high barrier height (BH), but with same ideality factor and series resistance as considered for a pure Schottky contact without an interfacial layer. This apparent BH decreases linearly with decreasing temperature. The effects giving rise to high ideality factor in metal–insulator–semiconductor diode are analysed. Reasons for observed temperature dependence of ideality factor in experimentally fabricated metal–insulator–semiconductor diodes are analysed and possible mechanisms are discussed.     

Effect of the Nickel Impurity on the Rate of Surface Generation of Charge Carriers at the Si–SiO2 Interface

The effect of the nickel impurity on the generation characteristics of the Si–SiO₂ interface is investigated by the method of isothermal capacitance relaxation. It is demonstrated that doping by the nickel impurity increases the rate of surface generation of charge carriers and the spread of this parameter from structure to structure. The effects observed are associated with the existence of defect-impurity complexes formed in doped structures as a result of interaction of impurity clusters (precipitates) with native defects in the transition layer at the Si–SiO₂ interface.     

Ni ultrathin film development on SrTiO3(1 0 0) surface

The Ni ultrathin film development on the SrTiO₃(1 0 0) surface doped by Fe acceptors (0.14 wt%) has been studied by AES, LEED, TDS, EELS and relative WF measurements. Heating of the clean STO surface above 1100 K causes TiO desorption. The adsorbate grows in the simultaneous multilayers (SM) mode, approaching a rather metallic character at high coverages. The nickel adatoms in the metal–oxide interface, interact with the outmost surface oxygen atoms resulting in a two-dimensional NiO compound. The formation of this surface oxide is: (i) time dependent, (ii) temperature independent for T>300 K and (iii) promoted by the presence of surface defects. A significant amount of Ni (1.4 ML) remains on the surface, strongly chemisorbed, after high temperature annealing.     

Experimental evaluation of interface states during time-dependent dielectric breakdown of GaN-based MIS-HEMTs with LPCVD-SiN_x gate dielectric

We experimentally evaluated the interface state density of Ga N MIS-HEMTs during time-dependent dielectric breakdown(TDDB). Under a high forward gate bias stress, newly increased traps generate both at the Si Nx/Al Ga N interface and the Si Nx bulk, resulting in the voltage shift and the increase of the voltage hysteresis. When prolonging the stress duration, the defects density generated in the Si Nx dielectric becomes dominating, which drastically increases the gate leakage current and causes the catastrophic failure. After recovery by UV light illumination, the negative shift in threshold voltage(compared with the fresh one) confirms the accumulation of positive charge at the Si Nx/Al Ga N interface and/or in Si Nx bulk, which is possibly ascribed to the broken bonds after long-term stress. These results experimentally confirm the role of defects in the TDDB of Ga N-based MIS-HEMTs.     

Analysis of frequency-dependent series resistance and interface states of In/SiO2/p-Si (MIS) structures

In this work, the investigation of the interface state density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G/ω−V) characteristics in In/SiO₂/p-Si metal–insulator–semiconductor (MIS) structures with thin interfacial insulator layer have been reported. The thickness of SiO₂ film obtained from the measurement of the oxide capacitance corrected for series resistance in the strong accumulation region is 220 Å. The forward and reverse bias C–V and G/ω−V characteristics of MIS structures have been studied at the frequency range 30 kHz–1 MHz at room temperature. The frequency dispersion in capacitance and conductance can be interpreted in terms of the series resistance (R_s) and interface state density (D_it) values. Both the series resistance R_s and density of interface states D_it are strongly frequency-dependent and decrease with increasing frequency. The distribution profile of R_s–V gives a peak at low frequencies in the depletion region and disappears with increasing frequency. Experimental results show that the interfacial polarization contributes to the improvement of the dielectric properties of In/SiO₂/p-Si MIS structures. The interface state density value of In/SiO₂/p-Si MIS diode calculated at strong accumulation region is 1.11×10¹² eV⁻¹ cm⁻² at 1 MHz. It is found that the calculated value of D_it (≈10¹² eV⁻¹ cm⁻²) is not high enough to pin the Fermi level of the Si substrate disrupting the device operation.     

Gated photoluminescence study of oxide-free InP MIS structure having an ultrathin silicon interface control layer

In order to investigate the effectiveness of a novel oxide-free surface passivation approach for InP, using an ultrathin silicon interface control layer (Si ICL), gated photoluminescence characteristics of the Si₃N₄/Si ICL/n-InP metal–semiconductor–insulator (MIS) structure were studied at room temperature. As compared with gated PL spectra of Si₃N₄/n-InP MIS without Si ICL, PL intensities of the sample with Si ICL were much more strongly modulated by the gate voltage. The interface state density distribution was estimated by an optical analog of the Terman’s C–V analysis and a good agreement with the C–V analysis was obtained. The result indicates complete removal of Fermi level pinning over the entire bandgap in the novel oxide-free MIS structure.     

The effect of potential of impedance in the pulsed-laser-deposited SrBi2Ta2O9 thin film

Ferroelectrics SrBi₂Ta₂O₉ (SBTO) thin films were grown on a highly oriented Pt/Ti/SiO₂/Si substrates using the pulsed laser ablation. The ac impedance of SBTO thin films have been measured at room temperature both in the frequency range from 10⁻¹ to 10⁶ Hz and bias voltage range from −6 to 6 V. The ac impedance dispersion was observed at low frequency with increasing bias voltage, which was interpreted based on a blocked charge. We can explain that the blocking interface gives rise to constant phase element (CPE) response, and we give an impedance model function that can fit data along the low frequency range when such a CPE is found. The low frequency dispersion phenomena of SBTO thin film are related to a charge diffusion process at the surface of thin film.     

Analysis of the photovoltaic efficiency of a molecular solar cell based on a two-level system

A molecular solar cell is modelled as a two-level system connected to electrodes by chains of electron-transporting and hole-transporting orbitals. Light absorption and emission are simulated using the generalised Planck equation and intermolecular charge transfer using non-adiabatic Marcus theory. Quantum efficiency–voltage characteristics, open-circuit voltage and monochromatic power-conversion efficiency are calculated as a function of the following parameters: charge-separation rate, interfacial recombination rate, charge mobility, light intensity and built-in bias. We find that slow charge separation, fast recombination and low mobility all contribute to a decrease in efficiency compared to the ideal (detailed balance) limit. When charge-separation and interfacial recombination rates are related through the intermolecular coupling, maximum efficiency is achieved at some optimum, but not the maximum, charge-separation rate. Two regimes are distinguished for the open-circuit voltage: when interfacial recombination is important, V_oc varies approximately linearly with the donor–acceptor energy gap; but when recombination is insignificant, V_oc is determined by the optical gap. Including exciton binding energy in the driving force for charge separation reduces V_oc. In systems with significant recombination, V_oc first increases and then saturates with increasing light intensity. Low mobility and interfacial recombination are the main avoidable sources of loss when realistic parameters are used, but the effects of low mobility can be partly compensated by applying a built-in bias between the electrodes. PACS 72.40.+w; 73.40.Lq; 72.80.Le     

The g–r noise in quantum well semiconductor lasers and its relation with device reliability

The g–r noise in quantum well semiconductor lasers is studied theoretically and experimentally. The results indicate that the g–r noise is dependent on bias current, the devices show the g–r a noise only at low bias current, with the bias current increasing, the g–r noise will disappear. The g–r noise has a close relation with defects; the devices with g–r noise degrade rapidly during the electric aging. It is means that measuring the noise at low bias current is very important for estimating device reliability.     

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: first-principles calculations

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.     

Study on typical behavior of transient nature (I-t) and hysterisis nature of I–V characteristics of dye doped solid state thin film photoelectrochemical cell

In this present investigation, we describe the steady state current voltage (I–V) characteristic of Crystal violet dye dispersed solid state photoelectrochemical cell (PEC). Typical behavior of dark current-voltage characteristic by increasing and decreasing external bias voltage has a similar form like hysterisis in nature. Although we have already observed this hysterisis nature in case of both forward and reverse bias condition, yet it is clear that the reverse hysterisis curve is more prominent than forward hysterisis. In this paper, we are getting double values of current (I) for a single value voltage, which is also helpful to understand the charge transport process through disordered materials. As the bias increases, the distribution of traps depth, which is exponential in nature, changes toward order state (resulting increase in disordered parameter α) This means that as α increases, it tends to reach the most order state of material. When external bias voltage is at 3.5 V, the value of disorder parameter becomes 1, and when bias voltage is beyond 3.5 V, the diffusion comes enhanced in nature.     

Low frequency noise in GaAs structures with embedded In(Ga)As quantum dots

Current–voltage and low frequency excess electrical noise characteristics of two different—Schottky diode and n-i-n diode—GaAs structures embedded with self-assembled In(Ga)As quantum dots are reported. We find the growth of quantum dots induces defects not only near the quantum dot but also extended to quite a distance toward the growth direction. In Schottky diode structure, comparing with the reference sample without the quantum dot layer, the current dependence of the low frequency noise spectral density indicated that the noise is from the generated interface states with the density increasing towards the band tail. Also the crystal quality of the Schottky diode including the quantum dot layer, deduced from the Hooge parameter, was slightly worse than that of the reference sample. For n-i-n diode structure, the current–voltage relation was linear, and a quadratic current dependence of the noise spectral density was observed. The Hooge parameter for the n-i-n structure was determined to be on the order of unity indicating the general degradation of the structure.     

Zero temperature phase diagram of the ferromagnetic Kondo lattice

We study numerically the one-dimensional ferromagnetic Kondo lattice, a model widely used to describe nickel and manganese perovskites. Due to the competition between double and super-exchange, we find a region where the formation of magnetic islands induces a charge-ordered state. This ordering is present even in the absence of any inter-site Coulomb repulsion and presents an insulating gap associated to the charge structure. We study the metal–insulator transition induced by a magnetic field which removes simultaneously both charge and spin orderings. This new mechanism should be taken into account in theories of charge ordering involving spin degrees of freedom.     

Photo-induced electrical instability in hydrogenated amorphous silicon based thin-film transistors and the effect of its phase transition

We investigate the origin of photo-induced electrical instability in hydrogenated amorphous silicon based thin-film transistors (a-Si:H TFTs). Photo instability alone was accompanied by a positive shift in the threshold voltage (V_TH) caused by photo irradiation, and even larger positive or negative shift further exacerbated the instability caused by photo-induced electrical bias stress. Such phenomena can occur as a result of extended charge trapping and/or the creation of defect-states at the semiconductor/dielectric interface or in the gate dielectric. The mechanism for such is difficult to describe through chemical interactions of electron-donating and -withdrawing molecules that exhibit a shift in V_TH in only one direction. We also prove that a transition from an amorphous to a protocrystalline phase improves the photo-induced electrical stability. Such results originate from a reduction in the density of the localized states in protocrystalline-Si:H films relative to that of a-Si:H. We believe that this study provides significant information on the device physics of optoelectronics, which commonly exhibit photo-induced instability and charge transport, as a result of prolonged exposure to photo irradiation.     

Electrical properties of nanointerface at poly(3-hexylthiophehe) and metal junctions probed directly with potential tip

Electrical properties at the nanointerfaces of head-to-tail coupled poly(3-hexylthiophene), PHT and metals (Au, Al) in sandwich type Al/PHT/Au diodes have been investigated using a nanomanipulator with the potential probing tip. It has been directly found that the highly insulating layer and the appreciable contact resistance are formed at Al/PHT with the thickness of several tens nanometer and PAT/Au, respectively. The bias dependence of the interface resistances at Al/PHT shows the origin of rectification. It has also been found that the interface resistance at PAT/Au is unexpectedly large, though the current–voltage behavior is ohmic. The results indicate the contact resistances between PHT and metals at the nanometric region are important factors to determine the diode performance.     

Multiple Andreev reflections in ballistic Nb-InGaAs/InP-Nb junctions

The current–voltage characteristics of ballistic Nb-InGaAs/InP-Nb Josephson junctions have been investigated. At temperatures below 1 K a negative differential conductance, which usually leads to a hysteresis in the current–voltage characteristics, was resolved by connecting an additional external shunt resistor to the junction. The negative differential conductance is explained by heating and conductance enhancement due to multiple Andreev reflections. The structures observed in the differential resistance measurements as a function of the bias voltage are explained by self-detection of Josephson radiation at low bias voltages and subharmonic gap structures at higher bias voltages.     

Charge transport in one‐dimensional chains of nanoparticles

Gold electrodes were bridged by a one‐dimensional chain of citrate stabilized gold particles. The resulting device exhibits current fluctuations at constant bias voltage due to conformational changes in the citrate molecules induced by charge transfer across the molecules. Moreover, fluctuations of the differential conductance as a function of the bias voltage became evident. The effect is attributed to the interplay of two origins: (a) dislocation of particles in the chain by action of the electric field; (b) interaction between mechanical motion of citrate molecules located on particles and charge transfer. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Charge-storage effects in a metal-insulator-semi-conductor structure containing germanium nano-crystals formed by rapid thermal annealing of an electron-beam evaporated germanium layer

Charge-storage effects in a metal-insulator-semi-conductor device containing germanium (Ge) nano-crystals were investigated. The Ge nano-crystals were formed by rapid thermal annealing (RTA) of an evaporated, ultra-thin Ge layer at 1000 °C in argon. Capacitance–voltage measurements shows that the amount of electrical charge which can be stored in the device varies with the duration of the RTA treatment. The charge shows a maximum value for 200 s RTA treatment, and then decreases with longer annealing time up to 400 s. Atomic force microscopy analysis indicates that there is a correlation between the density of Ge nano-crystals in the devices, and the amount of electrical charge stored. For an RTA treatment of 300 s, capacitance–time measurements show a time dependence, which indicates a dispersive carrier relaxation. The retention time is dependent on the applied bias, and a maximum retention time of 115 s was observed at -7 V. The value of the stored electrical charges in the device decreases with increasing ambient temperature. A possible charging/discharging mechanism for the device was discussed to explain the capacitance–time measurements and the temperature stored charge results. PACS 81.07.Ta; 81.15.Jj; 73.63.Kv; 85.35.Be; 81.40.Ef     

Direct observation of the potential distribution within organic light emitting diodes under operation

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq₃)/N, N ′‐bis(naphthalene‐1‐yl)‐N,N ′‐bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq₃ layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

True atomic resolution imaging of surface structure and surface charge on the GaAs(110)

We demonstrated a novel method to detect the van der Waals and the electrostatic force interactions simultaneously on an atomic scale, which is based on frequency modulation detection method. For the first time, the surface structure and the surface charge at atomic-scale point defects on the GaAs(110) surface have been simultaneously resolved with true atomic resolution under ultra-high vacuum condition. From the bias voltage dependence of the image contrast, we can verify that the sign of the atomically resolved surface charge at the point defect was positive.